It’s unclear who the donors are, or whether the bulk of the fundraising haul comes from mega donors, the way Mayor Mike Bloomberg aids his anti-gun effort. Americans for Responsible Solutions did not respond to requests about who gave contributions to the group.

“Our early success at building a powerful network of committed activists - and the strength of that network - shows that the issue of reducing gun violence in America isn’t going away,” said ARS executive director and co-founder Pia Carusone. “The hundreds of thousands of members and donors who’ve joined Gabby and Mark in just our first four months show that Americans know that we can protect the 2nd amendment AND make our communities safer. Americans want their elected officials to be honest and to lead with courage.”

So far, the group has only run a handful of ads — leaving the bulk of the money to help continue their efforts after the Senate failed to pass an expanded background checks last month.

Before the vote, Americans for Responsible Solutions went on the airwaves in Nevada, Kentucky, Ohio and California targeting the four Senate and House leaders. Following the background check vote, the group did four “thank you” ads applauding Sens. John McCain (R-Ariz.), Mary Landrieu (D-La.), Susan Collins (R-Maine) and Kay Hagan (D-N.C.) for their vote. They also did two attack ads hitting Senate Minority Leader Mitch McConnell (R-Ky.) and Sen. Kelly Ayotte (R-N.H.).